1. Field of the Invention
The present invention relates to an apparatus and a method for irradiating a subject to be irradiated with a laser beam and also relates to the beam homogenizer used for a laser beam irradiation apparatus.
2. Description of the Related Art
In order to form an integrated circuit with a thin film transistor (hereinafter referred to as a TFT) having high mobility over a glass substrate, an amorphous silicon film is deposited over the glass substrate by a CVD method or the like and the amorphous silicon film is applied with heat energy to be crystallized, so that a crystalline silicon film is formed. For crystallizing the amorphous silicon film, laser annealing is used to avoid transformation of the glass substrate by heat.
A crystalline structure of the crystalline silicon film crystallized by laser annealing depends on intensity of a laser beam. In a case where an amorphous silicon film is completely melted by laser beam irradiation, an interface between a solid phase (portion which is not melted) and a liquid phase (portion which is melted) moves in the silicon film at the same time as scanning with the laser beam, and thus crystal growth proceeds in a scanning direction and the crystalline silicon film having a large grain size can be formed. On the other hand, in a case where intensity of a laser beam is low and the amorphous silicon film is not completely melted but only a surface thereof is melted, numerous crystal cores are randomly generated at an interface between the silicon film and a base and crystals grow toward a surface of the silicon film from the crystal cores at the interface; therefore, the crystalline silicon film having a grain size smaller than that in the case where the amorphous silicon film is completely melted can be formed. When intensity of a laser beam is too low, the amorphous silicon film cannot be crystallized. When intensity of a laser beam is too high, the silicon film is ablated.
Therefore, in order to crystallize the amorphous silicon film by laser annealing, it is needed that the amorphous silicon film be applied with heat energy homogeneously with appropriate intensity by a laser beam. Under such conditions, the crystalline silicon film with a homogeneous crystalline structure and a large grain size can be formed over the glass substrate.
For improving throughput of laser annealing, a beam emitted from a laser oscillator is extended in one direction to be condensed in a direction perpendicular to an extension direction so as to be processed into a linear beam, by an optical system. Intensity distribution of the beam emitted from the laser oscillator is not homogeneous. For example, a laser beam of a single mode has Gaussian distribution in which intensity is higher toward the center of a curve. Even when the laser beam is processed into a linear shape, intensity distribution of the laser beam is still Gaussian distribution, and intensity of the ends of the laser beam is not high enough to melt amorphous silicon completely. Therefore, even when the amorphous silicon film is crystallized by such a linear laser beam, a periodical striped pattern occurs in the obtained crystalline silicon film due to a difference in the crystalline structure. By forming TFTs with such crystalline silicon films, each TFT has a different electrical characteristic due to a difference in the crystalline structure.
The inventor of the present invention developed a beam homogenizer for homogenizing intensity distribution of a laser beam (see FIG. 1 of Patent Document 1: Japanese Published Patent Application No. 2004-134785). The beam homogenizer shown in FIG. 1 of Patent Document 1 is described with reference to FIGS. 12A and 12B. FIGS. 12A and 12B are plan views of the beam homogenizer. FIG. 12B is a plan view of a plane seen from a direction perpendicular to that in FIG. 12A.
The beam homogenizer includes reflecting mirrors 11 and 12 which are rectangular and provided so that reflecting surfaces thereof face each other. A beam 13 which has entered the beam homogenizer propagates through the space between the reflecting mirrors 11 and 12 while being repeatedly reflected by the reflecting mirrors 11 and 12 as shown by the solid arrows and then is emitted from the beam homogenizer. On a plane P including an exit of the beam homogenizer, an irradiation region of the beam 13 is a region 14 corresponding to the exit of the beam homogenizer. When the beam homogenizer is not provided in a light path, the beam 13 propagates through a space as shown by the dotted arrows, and on the plane P, an irradiation region of the beam 13 is a region 15. That is, the beam 13 reaching regions 15a and 15b in the irradiation region 15 (the irradiation region when the beam homogenizer is not provided) is made to reach the irradiation region 14 by the beam homogenizer.
With the beam homogenizer, an incident beam is propagated while being reflected, thereby being split into a plurality of beams. All the split beams overlap each other at the exit and thus intensity distribution of the beam is homogenized.
Further, the inventor of the present invention developed an optical system of a laser irradiation apparatus for processing a laser beam into a linear shape and eliminating an adverse effect due to the fact that intensity distribution of the laser beam is Gaussian distribution (for example, Patent Document 2: PCT International Publication No. 2006/022196). In Patent Document 2, by making a laser beam pass through a slit so that the ends of the laser beam are shielded, a low intensity portion of the laser beam is avoided to be delivered. That is, in Patent Document 2, by an optical system including the slit, heat energy which is high enough to reliably carry out crystallization and to obtain a crystal with a large grain size is supplied to the amorphous silicon film.